The present invention relates to an ignition system for an internal combustion engine mainly applied to an automobile for controlling the dwell angle of an ignition coil at an optimum value.
In a conventional ignition system for the internal combustion engine of this type such as disclosed in U.S. Pat. No. 3,937,193, a signal generator generates an angle signal defining first and second sections of an ignition cycle with respective constant angles. An integrator circuit integrates in one direction during the first section and integrates in the other direction during the second section. An energization start signal generating circuit is enabled when the integration value of the integrator circuit reaches a predetermined value in the second section, and the primary current is cut off when the angle signal transfers from the second to first section thereby to generate a high ignition voltage on the secondary of the ignition coil.
In the above-mentioned conventional ignition system, however, assume that the angle signal of the signal generator transfers from the second to first section before the integration value of the integrator circuit reaches the predetermined value e.g. in the case of a sudden acceleration of the internal combustion engine. Under such a condition, a signal for turning off a power transistor is applied before the primary current flows in the ignition coil, with the result that an ignition high voltage fails to be generated in the ignition coil on the one hand and the integration is effected again in a direction from the midway integration value remaining in the integrator circuit on the other hand. As a consequence, the integration amount for the next cycle of integration in the other direction becomes larger than normally required, so that the time point when the integration value reaches the predetermined value in the second section, that is, the time point of start of the primary current is delayed, thereby causing a shortage of the ignition energy for the next ignition cycle.
Further, in the case where the engine speed is very low, the integration amount of the integrator circuit is limited in the second section to cause a shortage (saturation) of the integration amount of the integrator circuit of the second section. This causes the integration to reach a predetermined value earlier in the first section, resulting in an excessively long energization period.
To obviate this problem, a conventional system detects the engine speed, and when the detected speed is lower than a predetermined level, the energization start time point of the ignition coil is set to a fixed angular position (Japanese Patent Post-Examination Publication No. 53187/85). In this conventional system, however, it is necessary to provide a separate circuit for detecting the engine speed, and its construction is complicated.